5 basic for loop patterns

A for statement is used to execute a block of code repeatedly.

Three-component loop

This version of the Go for loop works just as in C or Java.

sum := 0
for i := 1; i < 5; i++ {
    sum += i
}
fmt.Println(sum) // 10 (1+2+3+4)

1. The init statement, i := 1, runs.

2. The condition, i < 5, is computed.

   • If true, the loop body runs,
   • otherwise the loop is done.
3. The post statement, i++, runs.
4. Back to step 2.

The scope of i is limited to the loop.

While loop

If you skip the init and post statements, you get a while loop.

n := 1
for n < 5 {
    n *= 2
}
fmt.Println(n) // 8 (1*2*2*2)

1. The condition, n < 5, is computed.

   • If true, the loop body runs,
   • otherwise the loop is done.
2. Back to step 1.

Infinite loop

If you skip the condition as well, you get an infinite loop.

sum := 0
for {
    sum++ // repeated forever
}
fmt.Println(sum) // never reached

For-each range loop

Looping over elements in slices, arrays, maps, channels or strings is often better done with a range loop.

strings := []string{"hello", "world"}
for i, s := range strings {
    fmt.Println(i, s)
}

0 hello
1 world

See 4 basic range loop patterns for a complete set of examples.

Exit a loop

The break and continue keywords work just as they do in C and Java.

sum := 0
for i := 1; i < 5; i++ {
    if i%2 != 0 { // skip odd numbers
        continue
    }
    sum += i
}
fmt.Println(sum) // 6 (2+4)

• A continue statement begins the next iteration of the innermost for loop at its post statement (i++).
• A break statement leaves the innermost for, switch or select statement.

Further reading

See 4 basic range loop (for-each) patterns for a detailed description of how to loop over slices, arrays, strings, maps and channels in Go.

Go step by step

Core Go concepts: interfaces, structs, slices, maps, for loops, switch statements, packages.

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